The two prevailing types of "prime movers" used to power machines are internal combustion engines and electric motors. The former are widely used for applications involving a machine which is mobile to the extent that it is impractical or impossible to provide electrical power to it. Examples include diesel locomotives, automobiles, ships and the like.
On the other hand, electric motors are used for stationary machines or for machines for which electrical power is readily provided even though such machines are, to some degree, mobile with respect to their surroundings. Examples of machines of the latter type include factory presses and material handling machines, e.g., overhead travelling cranes.
While overhead cranes travel on railroad-type rails suspended over the floor of, say, a factory, it is a relatively simple matter to provide electrical power to such cranes through a collector rail system. Such a system has plural, spaced, stationary horizontal bars mounted adjacent to the rails on which the crane rides and connected to an electrical power system. "Pick-up" shoes mounted on the crane slide along the bars and provide electrical power to the crane.
Many overhead travelling cranes are so-called "three-motion" cranes in that they have hoist, bridge and trolley drive motors. The hoist motor powers the drum for raising and lowering loads, the bridge motor powers the crane drive wheels for moving the crane along the rail and the trolley motor powers a trolley which moves on separate rails mounted on the bridge. Thus, such a crane can provide three axes of load movement, i.e., up/down (hoist), forward/reverse (bridge) and left/right (trolley). And cranes having two hoists or two trolleys are not all that uncommon. Four or even five drive motors may be involved.
Known overhead travelling cranes have a separate "dedicated" power supply for each motor. This arrangement permits any two or (if the operator is highly proficient and very careful) all three motors to be operated simultaneously. Of course, each power supply has to be made or otherwise procured by the crane manufacturer and the crane must be configured to provide a mounting space for each. And the weight of such power supplies must be taken into account when designing certain structural components of the crane.
Certain overhead cranes are used for light-duty service, e.g., Class C or Class D service as those classes are defined by the Crane Manufacturer's Association of America. Cranes for such light-duty service are used only occasionally. And when used, they are usually operated by a person having other primary job responsibilities. Such persons are often "novice" operators who use only a single crane "function" at any particular time. And for an occasionally-used light-duty crane, the time lost is not significant overall.
While occasional crane use and "one-function-at-a-time" operation have been recognized for some time as characterizing light duty cranes, no one (insofar as is now known) has recognized a disadvantage of such cranes. And if recognized, no solution is now known to have been put forth.
As further background, it is to be appreciated (as owners and users of light-duty cranes do) that such cranes are preferably built to the minimum standards and with the minimum equipment that will do the job. Such cranes are not production "tools" (rather, like a power house service crane, they are often used merely for maintenance-type functions) and their acquisition cost represents more of an expense than an investment in capital equipment used for production.
An improvement which reduces the cost and weight of a light-duty crane and which recognizes the operating requirements of such a crane would be an important advance in the art.